What makes a great accent wall?
The two main components of a successful accent wall are location and color choice.
Location of an accent wall.
The choice of where to place an accent wall is key and it's the number one mistake people make. The most successful accent wall is one which is already the focal point of the room. You are merely reinforcing that fact by adding the different color. It shouldn't be arbitrarily chosen. The types of walls which work well are those that are architecturally unique in the room like the freestanding wall above. It might have a fireplace on it, a gabled or cathedral shape, hold the main piece of art work or is the headboard wall in bedroom.
In the photo below the bright yellow wall holds the fireplace and is also the focus of the room due to its "A"shape. It's the natural place to use an accent color.
Choosing color for an accent wall.There are so many different color directions you can choose to go. It's hard to go wrong with a darker or brighter version of the color you are using on your other walls. It won't create a room that's too busy. Going lighter or more muted, on the other hand, will not maintain that wall as the room's main focus so it's not the best choice.
Black is a major trend in interior and exterior residential design these days and black makes a great color for a wall. An entirely black room can be overpowering for most people but it's very dynamic as an accent in an otherwise light colored room.
When the accent wall is just a few shades darker than the wall color it changes the overall effect. It creates a more soothing atmosphere as there is less contrast with the soft blend of colors.
Having the accent wall color come from a completely different hue family (color) than the rest of the walls is trickier to pull off. The first room above has accomplished this to great affect. In these scenarios it is most successful when the two colors are very different in terms of value (lightness/darkness) or saturation ( purity of color versus "grayed down"). In the room above, the yellow walls are much lighter than the navy blue. If they were both equally light, like pastels, it would be much less successful.
Accent walls which serve a function.
Accents walls in spaces like hospitals and offices are very common. In a waiting room they offer stimulation to keep a waiting patient visually occupied. In a hospital they do the same and also aid in "way finding" by accentuating areas of different function with different colors.
In homes, accent walls are more about decorating than about functionality. There are many options besides paint to create an accent wall. Millwork like paneling or shiplap and wallpaper come to mind. But all in all, a painted accent wall is an inexpensive way to create a design element in a room which adds to its uniqueness and beauty.